... operator in ES6

<script>    var foo=(...rest)=>{        var sum=0;        for(var i=0;i<rest.length;i++){            sum+=rest[i];        }        return sum;        }    foo(1,2,3);   //6    var boo=(a,...rest)=>{        var sum=0;        for(var i=0;i<rest.length;i++){            sum+=rest[i];        }        return sum;        }    foo(1,2,3);   //5    var arr1 = [1,2,3];    var arr2 = [4,5,6];    var arr3 = [7,8,9];    var Arr = [...arr1,...arr2,...arr3];    //[1,2,3,4,5,6,7,8,9]    var arr = [0,...arr1,...arr2,...arr3];    //[0,1,2,3,4,5,6,7,8,9]    var arr1 = [1,2,3];    var arr2 = [4,5,6];    var arr3 = arr1.push(...arr2);    //6    console.log(arr1);      //[1,2,3,4,5,6]</script>